In the so-called hydrotreating of hydrocarbons, wherein a hydrocarbon oil is hydrogenated, desulfurized, denitrogenated or decomposed in the presence of hydrogen, a catalyst composed of at least one metal selected from the metals of Group VI of the Periodic Table and the metals of Group VIII of the Periodic Table (the active component), such metals being carried on an inorganic oxide carrier such as alumina (Al.sub.2 O.sub.3), silica-alumina (SiO.sub.2 -Al.sub.2 O.sub.3), titania (TiO.sub.2) or the like, is employed. As the Group VI metal, molybdenum (Mo) and tungsten (W) are well utilized for the purpose, and as the Group VIII metal, cobalt (Co) and nickel (Ni) can be used.
Such a metal is generally carried on the carrier in the form of an oxide thereof, which, however, is an inactive compound. Accordingly, the catalyst must be activated by presulfurization so as to convert the metal oxide into the corresponding sulfide prior to being used in the hydrotreating reaction.
In a conventional technique for such presulfurization, a sulfurizing agent is introduced into the catalyst layer, together with hydrogen, after the catalyst has been filled in a reactor where hydrotreating a hydrocarbon oil is to be effected. The operating conditions for presulfurization vary in accordance with the hydrotreating process and with the sulfurizing agent used. For instance, where hydrogen sulfide is used as the sulfurizing agent, the compound is incorporated into hydrogen in an amount of from 0.5 to 5% by volume or so, and the hydrogen sulfide-containing hydrogen is applied to the catalyst in an amount of from 1000 to 3000 liters per liter of the catalyst (as calculated under the conditions of normal temperature and normal pressure) and the presulfurization is effected at 180.degree. C. or higher (generally 250.degree. C. or higher). Where carbon disulfide, normalbutyl mercaptan, dimethyl sulfide, demethyl disulfide or the like is used as the sulfurizing agent, the compound is diluted with a light hydrocarbon oil and is applied to the catalyst at a temperature of from 250.degree. C. to 350.degree. C., a pressure of from 20 to 100 kg/cm.sup.2, a liquid space velocity of from 0.5 to 2.0 hr.sup.-1, and a hydrogen/oil ratio of from 200 to 1000 N-liter/liter.
After completion of such presulfurization, the reaction system is substituted by a raw material oil which is actually to be processed and the intended hydrotreatment is started with the thus-activated catalyst. As the presulfurization has a significant influence on the success of the hydrotreatment to follow, pertinent selection of the materials to be employed in the process and careful operation of the process are required. Where a diluting agent is used, for instance, the agent must be a hydrocarbon oil free from olefins since olefins, if contained in the diluting agent, will form a polymer product and the product will poison the hydrotreating catalyst. The catalyst metal would be passivated when reacted at a high temperature with hydrogen to be reduced. Accordingly, in order to prevent such passivation, it is necessary to use an excess amount of the sulfurizing agent, and the proportion of the sulfurizing agent and hydrogen must be maintained properly. Further, although such presulfurization is generally carried out for several days, it is not automated in most cases as the operation is a temporary one. In addition, the presulfurization process requires some complicated treatment steps which are different from the conventional ones; thus, the process is labour intensive. For this reason, omission of the pre-sulfurization step or at least a reduction in the complexity of the presulfurization steps has been a desired goal in this technical field.
Recently, one method has been proposed which could meet the goal. This method is directed to presulfurization of an active metal-carried catalyst by impregnating the catalyst with a polysulfide of a general formula R-S(n)-R', where n represents an integer of from 3 to 20, and R and R' each represent a hydrogen atom or an organic group having from 1 to 150 carbon atoms per one molecule, and heat-treating the thus-impregnated catalyst at a temperature of 65.degree. to 275.degree. C. and a pressure of 0.5 to 70 bars and in the absence of hydrogen gas (Japanese Patent Application Laid-Open No. 61-111144).
In accordance with this method, the polysulfide as introduced into the catalyst sulfurizes the active metal by heat-treatment. Accordingly, where the above-mentioned presulfurization is carried out in a reactor, neither a sulfurizing agent nor a diluting agent is necessary and the operation is easy. In addition, the above-mentioned pre-sulfurization may also be effected even outside the reactor. In such a case, the presulfurized catalyst may be applied to the reactor, whereupon the intended hydrotreatment may immediately be started.
However, the amount of polysulfide used in the method is a stoichiometrical amount necessary for sulfurizing the whole active metal oxide (for example, CoO, MnO.sub.3) in the catalyst by the successive heat-treatment thereof, and the polysulfide is diluted in a pertinent organic solvent and then applied to the catalyst for impregnation. Since the polysulfide is highly viscous, the viscosity would still be high even though it is diluted with an organic solvent and, as a result, there would be a problem that the polysulfide would hardly penetrate into the inside of fine pores of the catalyst.
The catalyst to be treated by presulfurization is prepared by a method where an aluminium hydrate obtained from a starting material of sodium aluminate is shaped, dried and fired to give a .gamma.-alumina, and the resulting .gamma.-alumina is impregnated with an aqueous solution of a water-soluble compound of an active metal and then dried and heat-treated so that the active metal is converted into the corresponding oxide form, or by a method where an aluminium hydrate is blended with an aqueous solution of a water-soluble compound of an active metal and then shaped, dried and fired so that the active metal is carried on a carrier composed of the resulting .gamma.-alumina in the form of the oxide form of the active metal.